This invention relates to anodizing and electroplating and more particularly to a power supply which supplies current wherein a series of discrete positive current pulses are followed by a series of discrete negative current pulses. In the disclosed invention the cycle time, the ratio of positive to negative pulses, the width of the individual pulses, and the voltage of the pulses is adjustable.
In electroplating metals on a base member from an electrolyte by using direct current there are limitations on the speed of plating and the quality of the electrodeposited metal. It is well known in the prior art that for some metals, electrodeposition from an electrolyte upon a base member is improved by applying first a positive current to render the member cathodic, to deposit an increment of metal from the electrolyte, followed by a negative current of lesser value. Repetition of this cycle will build up for many metals a superior electrodeposit. That is, it has been found that in certain plating processes a more uniform coating of plating metal is achieved by periodically reversing the plating current so that some of the plated metal is periodically depleted from the base member.
Anodizing systems using both positive and negative current pulses have also been found to be advantageous for certain materials. Anodizing is defined as a process of forming oxide films on certain metals and alloys by electrolysis in suitable electrolytes. Essentially the process consists of applying an electric potential to a cell in which the metal being anodized is made the anode or positive electrode. The passage of current through the cell results in oxidizing conditions at the anode which converts the surface of the metal to the oxide. Under suitable conditions the metal on the surfaces transform to an adherent oxide.
Some of the first work done on hard anodic coatings on aluminum used cooled sulfuric acid and oxalic acid. The conditions were such that less aluminum was dissolved during anodizing and this resulted in a denser, less porous, hard deposit. It was found that the porosity of the anodic coating varied with the alloy composition. High strength aluminum formed with copper alloys, such as the 2000 series, would pit during anodizing due to the copper in the alloys.
It is desirable to have an anodizing power supply which permits anodizing of copper bearing aluminum alloys without severe etching.